1. Field of the Invention
The present invention relates generally to photovoltaic devices and, more particularly, to flexible and stretchable photovoltaic devices with surface-plasmon enhanced conversion efficiency.
2. Prior Art
The most readily available source of renewable energy is the sun. Solar energy is harnessed and converted directly into electrical energy by the use of photovoltaic (PV) devices. At the heart of a PV device is a semiconductor p-n junction which forms a photo diode. When the p-n junction is illuminated with light of the appropriate wavelength, an electron-hole pair is generated; the electron and the hole are pulled in opposite directions by the internal electric fields of the p-n junction. The resulting photo current may be used to drive an electrical appliance downstream such as a pocket calculator or a battery charger.
Most commonly, PV modules are made on crystalline silicon wafers. It is straightforward to fabricate a planar p-n junction by growing various layers with the required dopants, and to pattern the front current collecting electrode, usually in a fingered geometry. While a planar geometry is useful for such flat-area applications as rooftop solar panels, in some cases it is preferable to have PV devices which are flexible, or which can be fabricated on a curved surface, for instance to act as a functional electricity generating xe2x80x9cskinxe2x80x9d on portable devices such as laptops or cellular phones, or even car roofs and hoods, without giving up design aesthetics.
One way to achieve some amount of flexibility is by using amorphous semiconductors, which can be evaporated onto a flexible plastic substrate, as is disclosed in U.S. Pat. Nos. 4,663,828 and 4,663,829, to R. A. Hartman and P. E. Koch, respectively. A flexible device proposed by Texas Instruments Inc., is known as a Spheral device and is disclosed in U.S. Pat. No. 4,614,835 to K. R. Carson et al. The total yield of these Spheral devices is claimed to be close to 10%, which if accurate is impressive for devices that are not made of single-crystal silicon.
Spheral devices have semiconductor spheres lodged in apertures in a perforated metal electrode and are sandwiched between the perforated metal electrode and a second electrode. The spheres are comprised of an n-doped semiconductor and a p-doped semiconductor with one contacting the perforated metal electrode and the other contacting the second electrode. While these devices have a degree of flexibility, they are not stretchable and do not have a degree of flexibility needed to conform to the shape of surfaces used in many electrical devices.
Therefore it is an object of the present invention to provide a surface-plasmon enhanced photovoltaic device with increased energy conversion efficiency over the spheral photovoltaic devices of the prior art.
It is a further object of the present invention to provide a surface-plasmon enhanced photovoltaic device with increased flexibility over currently available photovoltaic devices.
It is yet a further object of the present invention to provide a surface-plasmon enhanced photovoltaic device with increased stretchability over currently available photovoltaic devices.
It is yet a further object of the present invention to provide a surface-plasmon enhanced photovoltaic device meeting the above objectives yet not suffering in yield.
It is still yet a further object of the present invention to provide a surface-plasmon enhanced photovoltaic device meeting the above objectives which can be easily and economically fabricated.
A surface plasmon enhanced photovoltaic device and method for its fabrication are provided. The surface plasmon enhanced photovoltaic device boosts the performance of commercially available photovoltaic devices such as those based on Texas Instruments Spheral technology in which the current-generating p-n junction is between two layers or shells of a silicon sphere. Spheres are lodged in a large array in apertures of a metallic front electrode of which the surface topography is such that incident (sun)light interacts resonantly with surface plasmons on the metal surface. This leads to an enhancement of the oscillating electric fields, and therefore of the effective light intensity, at the spheres. Since the overall device can be designed to be mechanically flexible, it may be applied as a power-generating skin to a device to be powered.
The surface-plasmon enhanced photovoltaic device of the present invention comprises: a first metallic electrode having an array of apertures, the first metallic electrode having an illuminated surface upon which light is incident and an unilluminated surface, at least one of the illuminated and unilluminated surfaces having an enhancement characteristic resulting in a resonant interaction of the incident light with surface plasmons on the surface; a second electrode spaced from the first metallic electrode; and a plurality of spheres corresponding to the array of apertures and disposed between the first metallic and second electrodes, each sphere having a first portion of either p or n-doped material and a second portion having the other of the p or n-doped material such that a p-n junction is formed at a junction between the first and second portions, an individual sphere being disposed in the apertures such that one of the first or second portions is in electrical contact with the first metallic electrode and the other of the first or second portions is in electrical contact with the second electrode.
The enhancement characteristic preferably comprises a periodic surface topography, such as holes, dimples, or surface corrugations, on at least one of the illuminated and unilluminated surfaces of the first metallic electrode, the apertures in the metallic electrode being of a diameter comparable to or less than a wavelength of light incident thereon.
In a preferred implementation, the first portion is an outer portion or shell and the second portion is a center portion or core of the spheres where the shell is in electrical contact with the first metallic electrode and the core is in electrical contact with the second electrode.
Also provided is an electrical device comprising: at least one component requiring a power supply; and a power supply for powering the at least one component. The power supply comprises the surface-plasmon enhanced photovoltaic device of the present invention. In a preferred implementation, the surface-plasmon enhanced photovoltaic device further comprises an under layer fixed to the second electrode and a pressure sensitive adhesive backing on a free surface of the under layer for applying the surface-plasmon enhanced photovoltaic device to an exterior surface of the electrical device.
Still yet provided is a method for fabricating a photovoltaic device. Wherein the photovoltaic device comprises: a first metallic electrode having an array of apertures; a second electrode spaced from the first metallic electrode; and a plurality of PV spheres corresponding to the array of apertures and disposed between the first metallic and second electrodes, each PV sphere having a shell of either p or n-doped material and a core having the other of the p or n-doped material such that a p-n junction is formed at a junction between the shell and core, an individual sphere being disposed in the apertures such that the shell is in electrical contact with the first metallic electrode and the core is in electrical contact with the second electrode. The method comprises the steps of: providing the first metallic electrode having the array of apertures; providing an array of the PV spheres, each PV sphere corresponding to an aperture in the first metallic electrode; depositing a photo resist on the shells; developing the photo resist to expose a portion of the shells; etching the exposed portion of the shells to expose a corresponding portion of the core; and depositing the second electrode on the exposed core.
The providing of the first metallic electrode having an array of apertures preferably comprises the sub-steps of: providing a substrate; forming a periodic array of first polymer spheres on the substrate; etching the first polymer spheres to form an array of etched polymer spheres, the etched polymer spheres being smaller than the first polymer spheres and having the same lattice constant; depositing a conductive film on the substrate and array of etched polymer spheres; and removing the array of etched polymer spheres resulting in the first metallic electrode being carried on the substrate.
The PV spheres are preferably fixed to the first metallic electrode with an adhesive to fill any gaps between the spheres in which case any adhesive from what will become the exposed portion of the shells must be removed. Furthermore, the etching step of the PV spheres preferably results in an overhang of the photo resist such that depositing the second electrode allows electrical contact to the core of the PV sphere while preventing electrical contact of the shell with the second electrode.